The trend is that in the future electronic systems which combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as wind power installations or solar installations for example, and also in vehicles, such as hybrid or electric vehicles.
In order to feed alternating current into an electrical machine, a DC voltage which is provided by a DC voltage intermediate circuit is usually converted into a three-phase AC voltage by means of a converter in the form of a pulse-controlled inverter. The DC voltage intermediate circuit is fed by a line of battery modules which are connected in series. In order to be able to meet the requirements for power and energy stipulated for a respective application, often a plurality of battery modules are connected in series in a traction battery. An energy storage system of this kind is often used, for example, in electrically operated vehicles.
The series circuit comprising a plurality of battery modules is associated with the problem that the entire line fails if a single battery module fails. Such a failure of the energy supply line can result in failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of an individual battery module can lead to power reductions in the entire energy supply line.
Document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this kind are known under the name Multilevel Cascaded Inverter or else Battery Direct Inverter (BDI). Systems of this kind comprise DC sources in a plurality of energy storage module lines, which can be connected directly to an electrical machine or an electrical power supply system. Single-phase or polyphase supply voltages can be generated in this case. The energy storage module lines in this case have a plurality of energy storage modules which are connected in series, wherein each energy storage module has at least one battery cell and an assigned controllable coupling unit, which makes it possible to interrupt the respective energy storage module line or to bridge the respectively assigned at least one battery cell or to connect the respectively assigned at least one battery cell into the respective energy storage module line, depending on control signals. By suitable driving of the coupling units, for example with the aid of pulse width modulation, suitable phase signals for controlling the phase output voltage can also be provided, with the result that a separate pulse-controlled inverter can be dispensed with. The pulse-controlled inverter required for controlling the phase output voltage is thus integrated into the battery.
As alternatives, DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclose battery cells in energy storage devices, which battery cells are connected in a modular manner and can be selectively coupled to and decoupled from the line of battery cells, which are connected in series, by means of suitably driving coupling units. Systems of this kind are known under the name Battery Direct Converter (BDC). Systems of this kind comprise DC sources in an energy storage module line, which DC sources can be connected to a DC voltage intermediate circuit for supplying electrical energy to an electrical machine or to an electrical power supply system by means of a pulse-controlled inverter.
In this case, the energy storage module line has a plurality of energy storage modules which are connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit which allows the in each case associated at least one battery cell to be bridged or allows the in each case associated at least one battery cell to be connected into the respective energy storage module line depending on control signals. The coupling unit can optionally be designed in such a way that it additionally allows the respectively associated at least one battery cell to also be connected into the respective energy storage module line or interrupt the respective energy storage module line with inverse polarity.
Both BDCs and BDIs usually have a higher degree of efficiency and a higher degree of fail-safety than conventional systems. The fail-safety is ensured, amongst other things, by defective battery cells, failed battery cells or battery cells which are not at full power being disconnected from the energy supply lines by suitable bridging driving of the coupling units.
When systems of this kind are used in electrically operated vehicles, the battery cells and/or other components of the respective energy storage modules may overheat and the energy storage device may be damaged. In order to avoid this, measures which keep the temperature of the energy storage device in a range which is not critical for the reliability and functioning of the energy storage device are usually initiated. Efficient, cost-effective and rapid-reaction methods for controlling the climate of modular energy storage devices are required.